There has been extensive investigation of the use of stimuli-responsive materials as industrially applicable functional materials. Among these, many organic materials, such as thermochromic leuco dyes and heat-sensitive liquid crystal materials, are in practical use for thermal response, i.e., response to heat, which is the most common form of stimulation.
Non-patent Literature 1 and 2 are a review on stimuli-responsive polymers and also mention graft polymers.
In addition, patent applications that have a technical feature of using stimuli-responsive polymers with a controlled structure have been filed. Patent Literature 1 discloses a polymer gel film made of a thermally responsive graft polymer. Patent Literature 2 discloses a composition that comprises an ABC-type block polymer including at least one stimuli-responsive block, and also discloses use of the composition as an ink composition, an image-forming method using the composition, and the like.
Further, Patent Literature 3 discloses that the use of an aqueous solvent, in particular, in the composition disclosed in Patent Literature 2, provides materials having excellent dispersion stability. Patent Literature 2 and 3 also disclose in the detailed description that the state of a system undergoes a “critical change” in response to stimuli, and disclose causing a temperature-induced sol-gel transition as an example of preferred change. Additionally, it is disclosed that the temperature that causes the critical change is about 20° C. in polyethoxyethyl vinyl ether and about 70° C. in polymethoxyethyl vinyl ether.
As a patent application that employs heat-thickening action of thermally responsive polymers, in particular, Patent Literature 4 discloses using a graft polymer in which one of a main chain segment and side chain segments has an LCST of 30 to 80° C. and the other is water-soluble, as a heat-thickening composition for coating of paper. Patent Literature 5 discloses an aqueous composition comprising a water-soluble polymer chain to which a polymer chain with an LCST is block-linked or grafted. Patent Literature 6 discloses a block copolymer thickener consisting of a hydrophilic polymer and a polymer having an LCST of 0 to 50° C. The compositions disclosed in Patent Literature 5 and 6 are used as cosmetics.
Meanwhile, Patent Literature 7 discloses a graft copolymer consisting of a hydrophilic main chain and a hydrophobic side chain. The graft copolymer disclosed in Patent Literature 7 is used as an aqueous ink pigment dispersion, and N-alkyl-substituted (meth)acrylamide is exemplified as a constituent monomer of the side chain. However, Patent Literature 7 nowhere discloses an LCST of the main chain and is also silent about thermal sensitivity and the function provided thereby.